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Vacuum Brazing of Stainless Steel and Dissimilar Materials

In the process of using stainless steel, considering the needs of corrosion resistance, heat resistance, oxidation resistance and other functions, it often involves vacuum brazing of stainless steel and dissimilar materials, such as vacuum brazing with dissimilar metals or ceramics.

1.Vacuum brazing of stainless steel and dissimilar metal materials

In recent years, the combination of stainless steel and low-density copper or aluminum alloy with good thermal strength and corrosion resistance has been more and more widely used, due to the difference in physical parameters such as melting point, linear expansion coefficient and specific heat capacity It is large, which produces a large thermal stress during the welding process. In addition, the presence of insoluble oxides brings more trouble to brazing. Aiming at these problems, researchers at home and abroad have paid great attention to the brazing of stainless steel and aluminum. The materials are shown in Table 1. Brazing in vacuum, first heating to 630C and holding for 10min can get very good joints.

In addition, the combination of high quality between stainless steel and titanium shows excellent performance under extremely harsh conditions such as high and low temperature environments, corrosive environments, etc., and has been systematically used in the aviation industry, electronics industry and chemical refining industry. The vacuum brazing of titanium and 304 stainless steel was discussed, and it was found that the brazing filler metal Ag-28Cu with eutectic composition was better than Ag-46Cu. The brazing temperature was 1 133 K, and the holding time was limited to 15 min, and good joint structure and properties were obtained under this process.

2.Vacuum brazing of stainless steel and ceramics

The high resistivity, high corrosion resistance and high temperature oxidation resistance of ceramics have attracted more and more attention and are being widely used in the fields of nuclear equipment, electronics, chemical industry and aerospace. Due to the low plasticity and high brittleness of ceramics, they are rarely used alone. To give full play to the advantages of ceramics, they must be combined with metals to complement their performance. However, the physicochemical properties of ceramics are so different from those of metals that the bonding of ceramics to metals becomes a challenging problem. The vacuum brazing of AbOs ceramics and SUS304 was studied, and 80N i20Cr was used as the brazing filler metal at 1 300 C, and satisfactory results were obtained. In addition, many people have studied the brazing characteristics of ceramics and stainless steel, and found that the Cu-Ag eutectic brazing filler metal cannot wet the ceramics well. ln also has the same effect, brazing at 950C, the joint strength is high.

3.Vacuum brazing process

Stainless steel can be brazed using any brazing method. Controlled atmosphere brazing of stainless steel is widely used, and the success of this brazing method is attributed to the development of reliable shielding gases and vacuum furnaces. The main requirements for stainless steel brazing equipment are that the furnace must have good temperature control (the deviation of the brazing temperature is required to reach ± 6C), and can quickly heat and accelerate cooling; all gases used in the controlled atmosphere furnace must be high. pure. The working vacuum degree of industrial vacuum brazing equipment is required to be between 1.33×10-3~ 13.3 Pa. The requirements for vacuum degree depend on the material to be welded, the brazing filler metal used, the area of the brazing joint, the amount of gas released from the base metal during the brazing process, and the leakage rate of the equipment.

At present, a pressure brazing process has been developed, such as brazing stainless steel with nickel-based solder, which can successfully achieve “zero” gap assembly. In addition to increasing the restraint strengthening effect, increasing the brazing pressure can also promote the diffusion of carbon, boron, silicon, especially silicon elements in the weld, thereby inhibiting the brittle phase of the weld and improving the strength of the brazed joint. However, the pressure should not be too high. When the brazing pressure is too high, the content of silicon and titanium in the interface area will increase, resulting in the formation of Cn, S and TCr brittle compounds, which will reduce the strength of the joint. With the development of vacuum heating equipment, an economical and practical combined brazing and heat treatment process has emerged in recent years, namely the integrated process of vacuum brazing and vacuum heat treatment. This process can not only give the joint almost the same mechanical properties as the base metal, but also make the part heated evenly, reduce deformation, the influence of harmful gases, etc. to a minimum, and maintain a bright surface without post-weld treatment.

The brazing process in the furnace has very strict requirements on the heating and heat preservation process. A little carelessness may cause the growth of grains and affect the performance of the base metal and the weld. However, induction brazing heating rates exceeding 100K/s can greatly reduce the deterioration of the base metal properties. The brazing of X-750 and SUS304 was studied by induction brazing, and it was found that the phenomenon of grain growth was not obvious when the shear strength reached 483 MPa at 1 423 K induction brazing. Provides assurance for the application of these materials in nuclear power plants and steam turbine turbines.

Brazing of large gap stainless steel is increasingly used in the production and maintenance of aero-engines. Aeroengines are mostly made of nickel-based superalloys and stainless steels with poor brazing properties, but the brazing of nickel-based alloys and stainless steels is greatly affected by the gap, and the gap is generally not more than 0.1 mm, otherwise a brittle phase will occur. It is very difficult to ensure a gap less than 0.1mm during engine maintenance, and the cost is also very high, so the large gap stainless steel brazing process is of great value. Unlike small-gap brazing, large-gap brazing cannot fill the gap by capillary action, so it is proposed to place alloy powder with a high melting point (called an additive) in the gap in addition to brazing filler metal. During the entire brazing process, the alloy powder remains insufficiently melted, providing a bridge for the capillary action of the brazing filler metal to facilitate the connection of the joint surfaces.

Recommended vacuum brazing equipment: RHVB series vacuum brazing furnaces produced by SIMUWU are high-quality products for vacuum heat treatment of tools and molds. Good temperature control accuracy and uniformity ensure the effective vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum annealing furnace, etc., which are widely sold in developed and developing countries.